(1) Field of the Invention
This invention relates to a method of fabrication used for semiconductor integrated circuit devices, and more specifically to an improved method for forming a tungsten plug on a contact hole.
(2) Description of Related Art
In the manufacture of semiconductor devices, it is necessary to make contact to device regions on the substrate surface through an overlying dielectric layer. This is accomplished by first forming an opening or via hole in the dielectric layer over the region to be contacted, and next filling the opening or via hole with a conductive material. As device geometries have shrunk to submicron dimensions and devices have become more densely packed on the substrate surface, the aspect ratio (ratio of height to width) of the openings or via holes to the device regions has greatly increased. This necessitates the use of CVD (Chemical Vapor Deposition) processes, such as LPCVD (Low Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition), to conformally deposit a refractory material, such as tungsten, into high aspect ratio openings or via holes and onto the substrate surface. CVD processes are effective in filling high aspect ratio openings or via holes because in these processes the conductive material growth occurs on both the vertical and horizontal surfaces and, therefore, filling of a narrow-deep contact hole is achieved when the deposited conductive material thickness on a horizontal surface is slightly greater than one/half the contact hole width. Tungsten is commonly used as the via hole filling material because of the ease of deposition of tungsten by CVD processes and the compatibility of tungsten with subsequently deposited aluminum or aluminum alloy interconnection metallization layers. A further improvement to contact technology is the deposition of a barrier and adhesion layer (or multilayer) of titanium and/or titanium nitride into openings or via holes in the dielectric layer and onto the surface of the dielectric layer prior to the deposition of the tungsten. This deposition coats the surface of the semiconductor at the bottom of the openings or via holes formed through the layer of dielectric material, as well as the walls of the openings or via holes formed through the dielectric layer and the surface of the dielectric layer. At the bottom of the via hole, the titanium and/or titanium nitride layer prevents excessive diffusion of the subsequently deposited tungsten into the semiconductor substrate and, also, produces a low contact resistance at the contact through the formation of a low resistance alloy with the silicon substrate at the metal-to-semiconductor interface.
Following CVD deposition of tungsten to fill the contact openings or via holes, a blanket (non-masked) plasma etch process is used to remove the tungsten from the surface of the dielectric layer while leaving plugs of tungsten within the contact openings or via holes. However, numerous problems exist when practicing blanket plasma etching of tungsten. For example, the uniformity of the tungsten etch processes is not ideal and tungsten residue commonly remains on the substrate surface, especially near the substrate edge, after the bulk of the tungsten has been removed from the substrate surface. Overetching in order to remove the tungsten residue causes undesirable etch attack of the tungsten at the contact sites and results in a reduction of the tungsten plug height, thus causing the tungsten plug to be recessed in respect to the top of the contact opening or via hole in the dielectric layer. Such a non-planar surface is difficult to reliably cover with subsequently deposited layers, such as aluminum or aluminum alloy metallization layers. Also, during the blanket etchback of the tungsten layer etching byproducts can redeposit on the substrate surface and result in "micro masking", which also produces undesirable residue. Overetching to remove the "micro masked" residue, also, results in excessive reduction of the tungsten plug height. Redeposition of etching byproducts also affects the manufacturability of the process because buildup of the redeposited byproducts on the processing chamber walls and other surfaces within the plasma etching apparatus increases the possibility of contamination of subsequently processed product. Also, small orifices within the processing chamber can become plugged with redeposited etching byproducts and result in degraded functionality of the plasma etching apparatus. For example, small orifices are used to direct helium onto the backside of substrates during plasma etching in order to produce cooling of the substrate during plasma etching. Helium is effective in this regard due to its high coefficient of thermal conductivity. If the orifices become plugged with redeposited byproduct, the flow of helium is impeded and the temperature of the substrate increases resulting in an undesirable change in etch rate. It is therefore desirable to have a tungsten etchback process which produces minimal etch byproduct redeposition because such a process requires a lower frequency of etching chamber cleaning. Necessity to frequently clean etch chambers adds to product cost because of decreased tool throughput and the added cost of performing the periodic maintenance and disposing of the cleaning waste.
Numerous improvements to the blanket tungsten etchback process have been invented. For example, U.S. Pat. No. 5,407,861 entitled "Metallization Over Tungsten Plugs" granted Apr. 18, 1995 to Maria S. Marangon et al describes a two step blanket etchback method for tungsten plugs in which the first step etches tungsten preferentially with respect to an adhesion layer, such as titanium-titanium nitride, and the second step etches etches tungsten and the adhesion layer at substantially equal rates.
Also, U.S. Pat. No. 5,397,742 entitled "Method for Forming Tungsten Plug for Metal Wiring" granted Mar. 14, 1995 to Choon H. Kim describes a combination of a blanket plasma etchback step and a wet etch step in order to reduce tungsten residues.
U.S. Pat. No. 5,521,121 entitled "Oxygen Plasma Etch Process Post Contact Layer Etch Back" granted May 28, 1996 to Chia S. Tsai et al describes a tungsten plug etchback process followed by an oxygen plasma treatment.
U.S. Pat. No. 5,164,330 entitled "Etchback Process for Tungsten Utilizing a NF.sub.3 /Ar Chemistry" granted Nov. 17, 1992 to Rickie L. Davis et al describes a three step blanket etchback method utilizing NF.sub.3 +Ar in the first step, NF.sub.3 +Cl.sub.2 +Al in the second step, and NF.sub.3 +Cl.sub.2 +Ar in the third step. The object is to reduce residue following the etch process.
While these inventions result in improvements to the blanket tungsten etchback process they do not address critical integrated circuit device manufacturability issues, such as etch byproduct production, overall process yield and process cost.